I modded an existing square top to fit my Nacra 5.2.
But I selected it so that the boom length was almost perfect to start with so I could simply remove a horizontal section and go sailing asap.
Worked well. Check the Texel rating sheets (floating out on the internet somewhere) for measurements, they will give you some idea of what kind of sails may be suitable.
Take note that the sails are either cut for floppy masts or masts with diamonds. Be sure to get a sail cut for a mast with diamonds.